66 Scientific American September 2018

By around 40000 years ago based on current

evidence H.  sapiens found itself all alone the only

remaining member of what was once an incredibly

diverse family of bipedal primates together known

as hominins. (In this article the terms “human” and

“hominin” both refer to H.  sapiens and its extinct

relatives.) How did our kind come to be the last hu-

man standing?

Until a few years ago scientists favored a simple

explanation: H.  sapiens arose relatively recently in

more or less its current form in a single region of

Africa and spread out from there into the rest of

the Old World supplanting the Ne and er tals and

other archaic human species it encountered along

the way. There was no appreciable interspecies frat-

ernizing just wholesale replac ement of the old

guards by the clever newcomer whose ascendancy

seemed inevitable.

Ye t mounting evidence from fossil and archaeo-

logical discoveries as well as DNA analyses has ex -

perts increasingly rethinking that scenario. It now

looks as though H.  sapiens originated far earlier

than previously thought possibly in locations across

Africa instead of a single region and that some of

its distinguishing traits—including aspects of the

brain— evolved piecemeal. Moreover it has become

abundantly clear that H. sapiens actually did mingle

with the other human species it encountered and

that interbreeding with them may have been a cru-

cial factor in our success. Together these findings

paint a far more complex picture of our origins than

many researchers had envisioned—one that privileg-

es the role of dumb luck over destiny in the success

of our kind.

THEORY UNDER THREAT

DEEATE AEOUT THE ORIGIN of our species has traditional-

ly focused on two competing models. On one side was

the Recent African Origin hypothesis championed by

paleoanthropologist Christopher Stringer and others

which argues that H. sapiens arose in either eastern or

southern Africa within the past 200000 years and

because of its inherent superiority subsequently re-

placed archaic hominin species around the globe with-

out interbreeding with them to any significant de-

gree. On the other was the Multiregional Evolution

model formulated by paleoanthropologists Milford

Wolpoff Xinzhi Wu and the late Alan Thorne which

holds that modern H.  sapiens evolved from Ne an de r-

tals and other archaic human populations through-

out the Old World which were connected through

migration and mating. In this view H. sapiens has far

deeper roots reaching back nearly two million years.

By the early 2000s the Recent African Origin mod-

el had a wealth of evidence in its favor. Analyses of the

DNA of living people indicated that our species origi-

nated no more than 200000 years ago. The earliest

known fossils attributed to our species came from two

sites in Ethiopia Omo and Herto dated to around

195000 and 160000 years ago respectively. And se-

quences of mitochondrial DNA (the tiny loop of genet-

ic material found in the cell’s power plants which is

different from the DNA contained in the cell’s nucle-

us) recovered from Ne an der tal fossils were distinct

from the mitochondrial DNA of people today—exactly

as one would expect if H.  sapiens replaced archaic hu-

man species without mating with them.

Not all of the evidence fit with this tidy story

however. Many archaeologists think that the start of

A

T THE DAWNING OF HOMO SAPIE

S OUR ANCESTORS WERE EORN INTO

a world we would find utterly surreal. It’s not so much that the

climate and sea levels or the plants and the animals were differ-

ent although of course they were—it’s that there were other

kinds of humans alive at the same time. For most of H.  sapiens’

existence in fact multiple human species walked the earth.

In Africa where our species got its start large-brained Homo

heidelbergensis and small-brained Homo naledi also roamed. In Asia there was Homo erectus

a mysterious group dubbed the Denisovans and later Homo floresiensis —a hobbitlike creature

tiny but for its large feet. The stocky heavy-browed Neandertals for their part ruled Europe

and western Asia. And there were probably even more forms as yet undiscovered.

IN BRIEF

Until recently the domi-

nant model of human

origins held that Homo

sapiens arose in a single

region of Africa and

replaced archaic human

species throughout the

Old World without inter-

breeding with them.

%yĀŠ ́mŸ ́‘åfrom

archaeology paleontol -

ogy and genetics are

rewriting that story.

The ̈DïyåïàyåyDà`›

suggests that H. sapiens

emerged from groups

located across Africa

and that interbreeding

with other human

species contributed to

our success.